In the fabrication of semiconductor integrated circuits on silicon wafers, chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD) forms dielectric or metallic films on the wafers at relatively low temperatures, such as 400° C. During the deposition process, the films are deposited not only on the wafers but also on the interior surface of CVD or PECVD reactor/chamber, in which the deposition process occurs. The deposition residue on the interior surface of the chamber can be a contamination source that may reduce the yield of subsequent deposition processes in the reactor. Thus, the residue is periodically removed.
The periodic removal of the deposition residue is performed by a chamber cleaning that uses a fluorine-based plasma. The reaction between the plasma and the deposition residue forms volatile products, which are evacuated from the chamber. For example, the fluorine-based chamber cleaning may be carried out after every fifty wafers have been processed. However, the fluorine-based plasma leaves sorbable contaminants and residues on the interior surface of the chamber. Normally, these contaminants and residues are removed only during primary maintenance, where the chamber is vented to the atmosphere and the interior surface of the chamber is physically scrubbed. Primary maintenance is performed far less frequently, e.g., after every 10,000 wafers.
The fluorine residue tenaciously adheres to the interior reactor surfaces of the CVD chamber and is extremely difficult to remove. Further, the fluorine residue can generate significant localized concentrations of HF when the chamber is opened to the atmosphere for the primary maintenance. Accordingly, the fluorine residue needs to be removed between the primary maintenance procedures, and a number of methods for removing fluorine residue have been developed.
The methods include purging the chamber with a hydrogen-containing or reducing gas, dissociating a reducing gas in a remote plasma source and then introducing it into the chamber, generating an H2 plasma in the chamber followed by purging the chamber with a dilute SiH4 mixture, generating a plasma of an inert gas in the chamber, and coating the interior surfaces of the CVD chamber with an oxide layer to trap the residual fluorine.
U.S. Pat. No. 5,647,953, which is incorporated herein by reference in its entirety, discloses a method for removing oxide and fluorine residues in a plasma process chamber. The method energizes fluorine containing gas into plasma and, using the plasma, removes oxide residue from interior surfaces of a CVD chamber. Then the interior surfaces are coated with silicon dioxide in order to trap fluorine impurities from the prior fluorine plasma-cleaning step. The silicon dioxide coating often leaves some of the fluorine impurities without the coating. Finally, a hydrogen-containing gas (SiH4, Si2H6, H2, and/or H2O) is introduced into the chamber in non-plasma state to remove the fluorine impurities uncoated in the prior coating step. This final step may be done as a purge, or the chamber may be pressurized and then evacuated.
U.S. Pat. No. 5,935,340, which is incorporated herein by reference in its entirety, describes a method for removing residues from the interior surfaces of a CVD chamber. This method dissociates a fluorine containing gas such as NF3 in a remote plasma source and then introduces the dissociated gas into the chamber to clean residues from the interior chamber surfaces. NH3, H2, and/or SiH4, are then introduced into the chamber to remove fluorine residue remaining from the prior step. In the removal of the fluorine residue, in order to cause a heat-induced chemical reaction between fluorine and NH3, H2, and/or SiH4 without in situ plasma excitation, a heater in the chamber maintains a temperature greater than 550° C. and less than about 600° C. NH3, H2, and/or SiH4 may first be dissociated in a remote plasma source before being introduced into the chamber. The clean reactants produced in the heat-induced chemical reaction, such as HF, SiF4, and/or an ammonium fluoride compound, are then evacuated from the chamber.
U.S. Pat. No. 5,824,375, which is incorporated herein by reference in its entirety, describes a method for removing sorbable contaminants from the interior surfaces of a chemical vapor deposition plasma reactor. This method includes cleaning the reactor with a plasma of a cleaning gas having a fluorine source that leaves sorbable contaminants and removing the sorbable contaminants with a plasma of a plasma of an inert gas, such as He. Then, a seasoning film is deposited on the interior reactor surfaces to block or retard remaining contaminants.
U.S. Pat. No. 5,129,958, which is incorporated herein by reference in its entirety, discloses a method of treating the fluorine residues in a CVD chamber left from a prior fluorine plasma cleaning step. This method contacts the fluorine residues with one or more reducing gases, such as SiH4, NH3, H2, PH3, B2Hs, and/or AsH3, to form one or more reaction products. The flow rate of the reducing gases is between 100 and 500 sccm (standard cubic centimeters per minute). While flowing the reducing gases, the CVD chamber is maintained at a temperature of 250 to 500° C. and a pressure of 10−3 to 100 Torr. The reducing gases react with the fluorine residues to form one or more gaseous and/or solid reaction products. Optionally at least a portion of the reaction products is removed from the CVD chamber.
U.S. Pat. No. 5,207,836, which is incorporated herein by reference in its entirety, describes a procedure for a removal of tungsten or tungsten silicide deposits from the susceptor of a vacuum deposition chamber by fluorine plasma cleaning and a removal of fluorine residues introduced by the plasma cleaning step. In this method, the tungsten-based residues are first removed by flowing a gaseous source of fluorine, such as SF6, NF3, C2P6, and/or CF4, into the chamber and igniting a plasma. Then, in order to remove the fluorine residue, a gaseous source of hydrogen, such as H2, B2H6, PH3, 1-2 carbon hydrocarbons, are flowed into the chamber at a rate of approximately 100 to 500 sccm, and a plasma is ignited. While the gaseous source of hydrogen is flowed, the pressure in the chamber is maintained at a pressure from about 0.2 to 1 Torr and the susceptor is maintained at a temperature from about 150 to 525° C.
U.S. Pat. No. 5,326,723, which is incorporated herein by reference in its entirety, discloses a method for cleaning a chemical vapor deposition chamber following tungsten deposition. After the tungsten deposition, an in-situ cleaning with an NF3 based plasma is used to remove tungsten residue from the chamber. This cleaning step produces fluoride-containing by-products. In the next step, an in-situ cleaning with an H2 based plasma is performed in the chamber. This plasma leaves hydrogen fluoride (HF), hydrogen (H), and fluorine (F) species in the chamber. Finally the chamber is purged with a mixture of SiH4, N2, and Ar at a pressure between 0.1 to 5 Torr, wherein the silane is between 1 and 2% of the mixture. The result of the last step is to replace a portion of the fluorine containing compounds on the interior chamber surfaces with SiF4 and H2. These gases (SiF4 and H2) and by-products are subsequently evacuated from the chamber.
U.S. Pat. No. 6,020,035, which is incorporated herein by reference in its entirety, describes a method for reducing the level of fluorine absorbed in films deposited on substrates in a PECVD (Plasma Enhanced CVD) chamber after a fluorine based chamber clean. In this method, a fluorine containing gas is introduced into the chamber to remove the deposition residue from the interior chamber surfaces. This cleaning process leaves a residue including fluorine atoms on the interior chamber surfaces. In the next step, a plasma of a process gas containing silicon and oxygen is used to deposit a silicon oxide film on the interior chamber surfaces. The plasma uses both low frequency (<2 MHz) and high frequency (>2 MHz) RF power sources, with the low frequency power source providing a power density greater than 3.10 W/cm2. The high power level of the low frequency RF signal increases the ion bombardment and favors the formation of stable SiF bonds between silicon and fluorine atoms. This leads to fewer loosely bonded fluorine atoms being incorporated into the layer and fewer fluorine atoms outgassing in subsequent process steps.
Although the prior art described above removes fluorine residue from CVD chambers, the effectiveness of fluorine residue removal process and the speed of the process still need improvement.